1989 325i making unearthly noise under the engine

Let me try to describe the sound.  It doesn't happen all the time,
just after a lot of driving.  Occurs when turning, especially on
uneven road surfaces.  It can happen when I brake, though I don't
think it's the brake pad sound.  It can continue even after I have
fully braked, and the car is standing still.  It can happen when I
decelerate, am close to braking.
It seems to come from the middle front, i.e., almost under the
airconditioning system.  Perhaps a little on the left, though it's
hard to tell.  It's not quite grinding, not quite clanging, not quite
rattling, not quite groaning/moaning, though a little of all of
these.  Definitely metallic.  The best I can describe is it sounds
like a plane going overhead, and I often have to look if there's a
plane, or a truck or something beside or behind me making the sound.
Recently the car was parked for almost 2 mos., during Houston 100
degree heat, at a stupid mechanic's.  The noise started after that.
I'd really appreciate help as to what this might be.  Before I take in
to another mechanic, not the same one.
Thank you!